The classic answer to your question is that if income tax is flat, then overall taxation is degressive, which most people consider unfair. There are taxes (and

The classic answer to your question is that if income tax is flat, then overall taxation is degressive, which most people consider unfair. There are taxes (and comparable unavoidable/imposed costs) related to many expenses. VAT is often mentioned, but is far from the only one. People with low income need to spend a higher fraction of their income than people whose income allows more saving, and thus pay more VAT viewed as a share of their income. There are many others. Schooling is mandatory in most countries and generally involves a cost per child imposed on the parents (because the public financing never covers 100% of the school's needs), so the costs imposed by the state as fraction of the parents' income is degressive. BTW, regarding that "unsustainable public spending"... if the interest rate the state pays is stable and low, then what makes the borrowing unsustainable? Surely the lenders' judgment of state finances is the one that matters.